a) Field of the Invention
The present invention relates to a pixel circuit and a light emitting display, and more particularly, to a pixel circuit and a light emitting display using the same, which emits light by a plurality of light emitting diodes coupled to one pixel circuit in order to improve the aperture ratio of the light emitting display.
b) Discussion of Related Art
In recent years, various display devices having reduced weight and volume compared to those of a cathode ray tube have been developed. In particular, light emitting displays having excellent light-emission, a wide angle of visibility, and a high-speed response have been proposed as next-generation planar type display devices.
A light emitting diode has a structure in which a light emitting layer emitting light is disposed between a cathode electrode and an anode electrode. Electrons and holes are injected from the cathode electrode and the anode electrode into the light emitting layer and are recombined to produce an exciton. When the exciton falls down to a lower energy level, light is emitted.
In such a light emitting diode, the light emitting layer may be composed of organic materials or inorganic materials. The light emitting diode may be an organic light emitting diode or an inorganic light emitting diode according to its material and structure.
FIG. 1 is a circuit diagram showing a part of an image display device in which a current programming type pixel circuit is used. Referring to FIG. 1, the image display device includes four pixels formed adjacent to each other. Each of the pixels includes an organic light emitting diode (OLED) and a pixel circuit. The pixel circuit includes a first transistor T1 through a fourth transistor T4, and a capacitor Cst. Each of the first transistor T1 through the fourth transistor T4 includes a gate, a source, and a drain. The capacitor Cst includes a first electrode and a second electrode.
The four pixels have the same structure. In an upper most left pixel, the first transistor T1 is coupled to the OLED and transfers a current for light emission to the OLED.
The amount of current transferred by the first transistor T1 is controlled by a data current applied through a second transistor T2. The data current is maintained for a predetermined time by a capacitor Cst coupled between a gate and a source of the first transistor T1.
A scan line Sn is coupled to gates of the second and third transistors T2 and T3. A data line Dm is coupled to a source side of the second transistor T2. A light emitting control line En is coupled to the gate of the fourth transistor T4.
Operation of the above-described pixel circuit will now be described. When a scan signal sn applied to gates of the second and third transistors T2 and T3 becomes low and the second and third transistors T2 and T3 are turned on, the first transistor T1 is diode-coupled and a voltage corresponding to a data current value Idata is stored in the capacitor Cst.
After the scan signal sn becomes high, the second and third transistors T2 and T3 are turned off, a light emitting control signal en becomes low, and the fourth transistor T4 is turned on, a power is supplied and a current from the first transistor T1 corresponding to a voltage stored in the capacitor Cst flows through the OLED to emit light. At this time, the current flowing through the OLED is expressed by the following Equation 1.
                    Idata        =                                            β              2                        ⁢                                          (                                  Vgs                  -                  Vth                                )                            2                                =                      I            OLED                                              (        1        )            
where Idata is a data current, Vgs is a voltage between the source and the gate of the first transistor T1, Vth is a threshold voltage of the first transistor T1, IOLED is a current flowing through the OLED, and β is a gain factor of the first transistor T1.
As indicated in Equation 1, although the threshold voltage Vth and a mobility of the first transistor T1 are non-uniform, since the current IOLED flowing through the OLED is identical to the data current Idata, uniform display characteristics can be obtained if a write current source of a data drive is uniform through the entire panel.
However, the current programming type pixel circuit mentioned above has a problem in that it takes a substantial amount of time to charge the data line since it should control a very small current. For example, assuming that a load capacitance of a data line is 30 pF, it takes a few milliseconds to charge a load of the data line with a current from several tens of nAs to several hundreds of nAs. Since a line time is only several tens of microseconds, there is not sufficient time to charge this load to the data line. In particular, when a low luminance is displayed, since a current value is small, a longer time is required to charge the load of the data line.
Furthermore, in a conventional pixel circuit in which a light emitting display is used, only one OLED is coupled to each pixel circuit. In order to emit a plurality of light emitting diodes, a plurality of pixel circuits are needed. Thus, the number of elements required within a light emitting display may be high.
Moreover, because one light emitting control line is coupled to each pixel row, the aperture ratio of a light emitting display may be deteriorated.